1. Field of the Invention
The present invention is related to a polymer dispersed liquid crystal display device, and more particularly to a polymer dispersed liquid crystal display device where a liquid crystal is dispersed in a liquid crystal polymer medium.
2. Description of the Related Art
Generally, in the various liquid crystal (LCD) devices, those using twisted nematic (TN) mode have low operation voltage and low power consumption, and have been widely used in recent years. The TN mode cell has a liquid crystal layer interposed between first and second substrates on which transparent electrodes are formed. The optical axes of liquid crystal molecules are parallel to the planes of the substrates. The first and second substrates are arranged at a twist angle of almost 90.degree.. However, since polarization system is used in the TN mode LCD device, there is a problem that the efficiency of light is low.
To solve the problem, a polymer dispersed liquid crystal display (PDLCD) device in which liquid crystal droplets are dispersed in a polymer medium, is proposed. In the PDLCD device, liquid crystal molecules are arranged randomly when a voltage is not applied to cell, and incident light is scattered by the difference in refraction indices between the liquid crystal and the polymer. As a result, a milky white state exists. On the other hand, when a voltage is applied to the cell, the liquid crystal molecules are arranged to be parallel to electric field, and incident light is transmitted because of no the difference in refraction indices between the liquid crystal and the polymer. As a result, a transparent state exists. Thus, since incident light is switched by light scattering without using the polarization system, there is a significant increase in the efficiency of light.
Eq. 1 shows refraction index n(a) of the liquid crystal at any view angle. EQU n(a)=n.sub.e n.sub.o /(ne.sup.2 cos.sup.2 a+n.sub.o.sup.2 sin.sup.2 a).sup.1/2 Eq. 1
wherein a is a view angle, n.sub.e is an index of refraction for extraordinary light, and n.sub.o is an index of refraction for ordinary light.
As shown in Eq. 1, the liquid crystal has birefringence index .DELTA.n where the refractive index varies with view angle, while the polymer does not have it. Therefore, refraction index makes a difference in the interface between the polymer medium and liquid crystal droplet, so that incident light is scattered at the interface between the polymer medium and liquid crystal droplet at even a transparent state. As a result, transmittance decreases at right and left view angle. Furthermore, the durableness of the device deteriorates by weak bonding strength of the polymer medium durableness of device.